LT6557

9

6557fa

IN

2.5k

9.1k

6557 F01

I =

Figure 1. Simplified Programmable Input Bias Circuit Diagram

no-signal amplifier input bias condition according to the

following relationship:

V

Vk

R

BIAS IN

PIN

SET

()

•.

=

For single 5V supply operation, a 400Ω programming

resistor is generally optimal. In applications that demand

maximum amplifier linearity, or if external biasing is

preferred (in DC-coupled applications, for example), the

internal biasing circuitry may be disabled by leaving Pin

16 open. With Pin 16 open, input loading is approximately

200kΩ.

Shutdown Control

The LT6557 may be placed into a shutdown mode, where

all three amplifier sections are deactivated and power sup-

ply draw is reduced to approximately 10µA. When the EN

pin is left open, an internal 40k pull-up resistor brings the

able the LT6557 (see Figure 2 for equivalent circuit). The

pull-down current required to activate the part is typically

125µA. In most applications, the EN pin is simply con-

nected to ground (for continuous operation) or driven

directly by a CMOS-level logic gate (see Figure 3 for

examples). Response time is typically 50ns for enabling,

and 1µs for shutdown. In shutdown mode, the feedback

resistors remain connected between the output pins and

40k

BIAS

CIRCUITRY

EN

Figure 2. Simplified Shutdown Circuit Diagram

LT6557

2

(3a) Open Drain or Open Collector

1

EN

DISABLE

LT6557

2

6557 F03

(3b) CMOS Gate with Shared Supply

1

EN

DISABLE

Figure 3. Suitable Shutdown Pin Drive Circuits

Programmable Input Bias

The LT6557 contains circuitry that provides a user-pro-

grammed bias voltage to the inputs of all three amplifier

sections. The internal biasing feature is designed to mini-

mize external component count in AC-coupled applica-

tions, but may be defeated if external biasing is desired.

Figure 1 shows the simplified equivalent circuit feeding

the noninverting input of each amplifier. A programming

resistor from Pin 16 to GND (Pin 2) establishes the nominal

APPLICATIONS INFORMATION